1. Field of the Invention
The present invention relates in general to electrochromic devices, and more particularly, to normally operating, color-stabilized electrochromic devices having an electrochromic medium comprising one or more additives, which serve to substantially preclude the formation of undesirable residual color within the electrochromic medium while in its high transmission state.
2. Background Art
Electrochromic devices have been known in the art for several years. While the utilization of electrochromic devices, such as electrochromic mirrors, has become increasingly popular among, for example, the automotive industry, the development of undesirable residual color within the electrochromic medium remains problematic.
Indeed, when a sufficient electrical potential difference is applied across the electrodes of a conventional device, the electrochromic medium becomes intentionally colored (i.e. a low transmission state) inasmuch as one or more of the anodic and the cathodic materials are oxidized and reduced, respectively. Specifically, the anodic materials are oxidized by donating electrons to the anode, and the cathodic materials are reduced by accepting electrons from the cathode.
For most commercially available devices, when the electrical potential difference is removed or substantially diminished, the anodic and cathodic materials return to their native or unactivated state, and in turn, return the electrochromic medium to its colorless or nearly colorless state (i.e. a high transmission state). The application and removal of an electrical potential difference is conventionally known as a single cycle of the electrochromic device.
Scientists have observed that over a period of cycles and/or time, during normal operation of the electrochromic device, the electrochromic medium sometimes does not remain colorless in the high transmission state. In some instances, even in the absence of an electrical potential difference, either one or both of a portion of the anodic and cathodic materials are oxidized or reduced respectively, thereby forming residual oxidized and/or reduced materials. The residual oxidized anodic materials and/or the residual reduced cathodic materials of the electrochromic medium can result in an undesired residual coloration of the electrochromic medium.
Factors that are believed to facilitate the formation of the undesired residual oxidized anodic and/or reduced cathodic materials include, among other things, impurities within the medium, thermal and/or photochemical decomposition of one or more of the medium materials, and/or the permeation of water and/or oxygen into the electrochromic medium.
It is therefore an object of the present invention to provide an electrochromic medium with a color-stabilizing additive that remedies the aforementioned detriments and/or complications associated with maintaining a colorless or nearly colorless electrochromic device while the device is in its high transmission state.
The present invention is directed to an electrochromic device, comprising: (a) a first substantially transparent substrate having an electrically conductive material associated therewith; (b) a second substrate having an electrically conductive material associated therewith; (c) an electrochromic medium contained within a chamber positioned between the first and second substrates which comprises: (1) a solvent; (2) a cathodic electroactive material; (3) an anodic electroactive material, wherein at least one of the anodic and cathodic electroactive materials is electrochromic; and (4) a color-stabilizing additive, wherein the color-stabilizing additive is electroactive within a normal operating voltage of the electrochromic device; and (d) means associated with the color-stabilizing additive for maintaining an at least nearly colorless electrochromic device while the electrochromic device is in an open circuit and/or zero potential state relative to the same device without the color-stabilizing additive.